Syntax: +
+compute ID group-ID vacf ++
Examples: +
+compute 1 all vacf +compute 1 upper vacf ++
Description: +
+Define a computation that calculates the velocity auto-correlation +function (VACF), averaged over a group of atoms. Each atom's +contribution to the VACF is its current velocity vector dotted into +its initial velocity vector at the time the compute was specified. +
+A vector of four quantites is calculated by this compute. The first 3 +elements of the vector are vx * vx0 (and similarly for the y and z +components), summed and averaged over atoms in the group. Vx is the +current x-component of velocity for the atom, vx0 is the initial +x-component of velocity for the atom. The 4th element of the vector +is the total VACF, i.e. (vx*vx0 + vy*vy0 + vz*vz0), summed and +averaged over atoms in the group. +
+The integral of the VACF versus time is proportional to the diffusion +coefficient of the diffusing atoms. +
+IMPORTANT NOTE: If you want the quantities calculated by this compute +to be continuous when running from a restart file, +then you should use the same ID for this compute, as in the original +run. This is so that the created fix will also have the same ID, and +thus be initialized correctly with atom velocities from the restart +file. +
+Output info: +
+This compute calculates a global vector of length 4, which can be +accessed by indices 1-4 by any command that uses global vector values +from a compute as input. See this +section for an overview of LAMMPS output +options. +
+The vector values are "intensive". The vector values will be in +velocity^2 units. +
+Restrictions: none +
+Related commands: +
+ +Default: none +
+ diff --git a/doc/compute_vacf.txt b/doc/compute_vacf.txt new file mode 100644 index 0000000000..b397d9945b --- /dev/null +++ b/doc/compute_vacf.txt @@ -0,0 +1,65 @@ +"LAMMPS WWW Site"_lws - "LAMMPS Documentation"_ld - "LAMMPS Commands"_lc :c + +:link(lws,http://lammps.sandia.gov) +:link(ld,Manual.html) +:link(lc,Section_commands.html#comm) + +:line + +compute vacf command :h3 + +[Syntax:] + +compute ID group-ID vacf :pre + +ID, group-ID are documented in "compute"_compute.html command +vacf = style name of this compute command :ul + +[Examples:] + +compute 1 all vacf +compute 1 upper vacf :pre + +[Description:] + +Define a computation that calculates the velocity auto-correlation +function (VACF), averaged over a group of atoms. Each atom's +contribution to the VACF is its current velocity vector dotted into +its initial velocity vector at the time the compute was specified. + +A vector of four quantites is calculated by this compute. The first 3 +elements of the vector are vx * vx0 (and similarly for the y and z +components), summed and averaged over atoms in the group. Vx is the +current x-component of velocity for the atom, vx0 is the initial +x-component of velocity for the atom. The 4th element of the vector +is the total VACF, i.e. (vx*vx0 + vy*vy0 + vz*vz0), summed and +averaged over atoms in the group. + +The integral of the VACF versus time is proportional to the diffusion +coefficient of the diffusing atoms. + +IMPORTANT NOTE: If you want the quantities calculated by this compute +to be continuous when running from a "restart file"_read_restart.html, +then you should use the same ID for this compute, as in the original +run. This is so that the created fix will also have the same ID, and +thus be initialized correctly with atom velocities from the restart +file. + +[Output info:] + +This compute calculates a global vector of length 4, which can be +accessed by indices 1-4 by any command that uses global vector values +from a compute as input. See "this +section"_Section_howto.html#howto_15 for an overview of LAMMPS output +options. + +The vector values are "intensive". The vector values will be in +velocity^2 "units"_units.html. + +[Restrictions:] none + +[Related commands:] + +"compute msd"_compute_msd.html + +[Default:] none